In the manufacture of 141b and 142b by the hydrofluorination of either vinylidene chloride or 140a, the reactor effluent generally contains hydrochlorofluorocarbons and unreacted hydrogen fluoride. The latter arises from feeding hydrogen fluoride in excess of its stoichiometric requirement to the reaction mixture. This is typically the case, as it is usually necessary to employ a large excess of hydrogen fluoride in the hydrofluorination reaction. It is highly desirable to recover the excess hydrogen fluoride for recycle to the reaction mixture.
The principle halohydrocarbon components of the reaction product mixture are 141b and 142b. If 140a is the product precursor, then the reactor effluent will also contain a substantial amount of HCl.
Hydrogen fluoride and 142b form an azeotrope. They cannot therefore be separated by simple distillation. Thus, U.S. Pat. No. 4,209,470 describes a process for separating hydrogen fluoride and 142b by decantation, which may be used for separating liquid mixtures of hydrogen fluoride and 142b generated in the hydrofluorination of 140a or vinylidene chloride. The quality of phase separation is enhanced by the addition to the liquid mixture, of an amount of a halocarbon selected from the group of 141b, vinylidene chloride and 140a. A liquid organic phase enriched in 142b and a liquid inorganic phase enriched in HF are formed. The HF-containing inorganic phase is then subjected to azeotropic distillation. The distillation bottom stream, comprising hydrogen fluoride, is recycled to the hydrofluorination reactor feed.
Japanese Patent Application No. 48/32449 teaches a similar separation wherein the halocarbon added to the HF/142b mixture subject to separation comprises 140a.
The process of U.S. Pat. No. 4,209,470 relies on the generation of an HF recycle stream derived from the phase separation of hydrogen fluoride and 142b. Because these molecules have such high mutual solubility, contamination of the HF recycle stream with 142b is likely. The return of even a small amount of 142b to the hydrofluorination reaction mixture, particularly where the object of the hydrofluorination reaction is the production of 141b, would have an adverse effect on the product distribution, promoting the formation of more fluorinated products, such as 1,1,1-trifluoroethane (hereinafter "143a").
What is needed is a process for separating hydrogen fluoride, 141b and 142b from their liquid mixtures generated from the hydrofluorination of 140a or vinylidene chloride. The process should permit the recycling of highly pure, substantially 142b-free hydrogen fluoride to the hydrofluorination reaction mixture.